Blood vessel occlusions are commonly treated by mechanically enhancing blood flow in the affected vessels, such as by employing a stent. Stents act as scaffolding, functioning to physically hold open and, if desired, to expand the wall of affected vessels. Typically stents are capable of being compressed, so that they can be inserted through small lumens via catheters, and then expanded to a larger diameter once they are at the desired location. Examples in the patent literature disclosing stents include U.S. Pat. No. 4,733,665 issued to Palmaz, U.S. Pat. No. 4,800,882 issued to Gianturco, and U.S. Pat. No. 4,886,062 issued to Wiktor.
FIG. 1 illustrates a conventional stent 10 formed from a plurality of struts 12. The plurality of struts 12 are radially expandable and interconnected by connecting elements 14 that are disposed between adjacent struts 12, leaving lateral openings or gaps 16 between adjacent struts 12. Struts 12 and connecting elements 14 define a tubular stent body having an outer, tissue-contacting surface and an inner surface.
Stents are used not only for mechanical intervention but also as vehicles for providing biological therapy. Biological therapy can be achieved by medicating the stents. Medicated stents provide for the local administration of a therapeutic substance at a diseased site. Local delivery of a therapeutic substance is a preferred method of treatment because the substance is concentrated at a specific site and thus, smaller total levels of medication can be administered in comparison to systemic dosages that often produce adverse or even toxic side effects for the patient.
One method of medicating a stent involves the use of a polymeric carrier coated onto the surface of the stent. A composition including a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend is applied to the stent by immersing the stent in the composition or by spraying the composition onto the stent. The solvent is allowed to evaporate, leaving on the stent surfaces a coating of the polymer and the therapeutic substance impregnated in the polymer.
A shortcoming of the above-described method of medicating a stent is the potential for coating defects due to the nature of the composition applied to the stent. For solvents that evaporate slowly, or “non-volatile” solvents, the liquid composition that is applied to a relatively small surface of the stent can flow, wick and collect during the coating process. As the solvent evaporates, the excess composition hardens, leaving clumps or pools of polymer on the struts or “webbing” between the struts. For solvents that evaporate very fast, or “volatile solvents,” the coating can be rough with a powder like consistency.
For slow evaporating solvents, heat treatment has been implemented to induce the evaporation of the solvent. For example, the stent can be placed in an oven at an elevated temperature (e.g., 60 deg. C. to 80 deg. C.) for a duration of time, for example, at least 30 minutes, to dry the coating. Such heat treatments have not reduced pooling or webbing of the polymer. Moreover, prolonged heat treatment can adversely affect drugs that are heat sensitive and may cause the warping of the stent. The manufacturing time of the stent is also extending for the time the stent is treated in the oven.
An apparatus and method is needed to address these problems. The embodiments of this invention address these and other problems associated with coating stents.